Combined grading and corona shield



April 10, 1956 R. L. MCCOY ET AL 2,741,321

COMBINED GRADING AND CORONA SHIELD Filed April 24, 1953 o o I Inventors:

R be t: 1.. Mc Cog, Isaac W Gross,

The iv Abtorneg.

United States Patent COMBINED GRADING AND CORONA SHIELD Robert L. McCoy, Baltimore, Md., and Isaac W. Gross, Glen Head, N. Y., assignorsto General Electric Com pany, a corporation of New York Application April 24, 1953, Serial No. 350,793 Claims. (01. 174-144 This invention relates to electrostatic shielding and more particularly to improvements in the electrostatic shielding'of-the hardware at line potentifl which supports a high .voltage alternating current power transmission line.

By high voltage is meant an alternating voltage, of between 300 and 500 kilovolts at commercial frequencies. lt is well known that corona discharge takes place in air from electrically conducting parts or electrodes at high voltage-and that the corona startingvoltage-is lower with parts having abrupt changes in curvature or sharp edges compared'with smoothly contoured curved parts. -It' is .also well known that when :high voltage conducting .parts are insulated from ground by a string of insulators that capacitance eiiects, to adjacent grounded structures causes the voltage to distribute itself unequally along thestring 10f insulators with the greatest stress or potential gradient at the-line or high potential end of the string. It is addi= tionally well known that appropriately positioned smooth- :ly contouredshielding members attached to the high voltage parts will eliminate corona discharges and also will grade the stress in an insulator string so as to bring the :electrical stress in the insulator or insulators at the line end of thestring more nearly into equality or balance with the stress of the other insulators of the string.

Heretofore, such shielding means have been relatively difficult to mount on transmission line parts. when the transmission line is energized and it has usually been necessary to'disconnect the cable conductor of the line from. its supporting parts in order to install or remove the shield. :Furthermore, in many installations the shield .has beenattached directly to the cable clamping member which adds to the inertia of the cable and clamp assembly vandresults in undesirable vibration effects. Furthermore, most of the prior art shields would act either ,to. grade the-stress distribution in an insulator string or. to control corona but not to perform both functions.

In accordance with the present invention, there is provided a novel electrostatic shielding arrangement which may readily be installed on or removed from a line with of the line from its supports. in addition the novel shielding ,-arran gement which is the subject matter of this invention is constructed to grade the stress distribution in a stringof insulators and at the same time to control corona discharge from edges of a cable clamp attached to the string of insulators. Furthermore, the shielding arrange ment isso constructed that it is attached to the conne'ct'ing means between a string of insulators and a cable clamp so that the mass of the shield does not add to the inertia of. the cable clamp combination.

Anobjectof the invention is to provide a new and improved electrostatic shielding arrangement. 7 Another object of the invention is to provide anew and improved shieldingarrangement for the hardware supp o'rti ng a' high voltage transmission line.

and improved grading shield'arrangement for both strain- 2,741 ,321 Patented Apr. 10, 1956 type'and suspension-type insulator strings for high voltage transmission lines.

An additional object of the invention is to provide a new and improved combination grading and corona shield for quick detachable connection to the line end of a suspension insulator string for high voltage transmission lines. I The invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appendedclaims.

In the drawing Fig. 1 is a perspective view of the in vention as applied to a pair of double-strain insulator strings and a single suspension insulator string attached to anendof a cross arm of a metal supporting towerfor high voltage transmission line; Fig. 2 is a side elevation view of-the structure at the line end of the suspension insulatorstring shown in Fig. l; Fig-3 is an end elevation partly in section of the structure viewed in the direction of the arrows 3-3 shown in Fig. 2; Fig. 4 is an end view along the line of arrows 44 of Fig. 5, partly in section,

of the. shielding structure at the line end of one of the double-strain insulator strings shown in Fig. l; and Fig. 5 is a side elevation partly in section viewed along the line of arrows 5-5 of Fig. 4. p 7

Referring now to the drawing and more particularly to Fig. 1, there is shown therein a portion of a structural steel cross arm 1 on a high voltage transmission line sup,- port tower- (not shown). Attached to the outerend of the arm 1 are two double-strain insulator strings and a single suspension insulator string 3. The individual insulators of each string may be of any suitable suspension type, such as the well known cap and pin type, in which each insulator has a socketon one side and a ball stud on the other side so that they may be connected together in astring by inserting the ball end of one into the socket of another. As shown, the double-strain insulator strings are attached to the end of the cross arm 1 by means of yoke. sets 4. A transmission line cable conductor 5 is attached to the line end of each double-strain insulator string by means of a dead end clamp 6 which is shown as of the compression type. The interconnecting means between each dead end clamp and the double-strain insulator string being completed through a yoke set 7 and clevis-type strain connector 8 whereby the clamp 6 has freedom of movement relative to the yoke set and lin end of the double-strain insulator string.

' The two dead end clamps 6 are interconnected by a jumper connection in the form of a cable conductor 9 whose'ends terminate in suitable clamps 10 which are customarily of the compression type for connecting to the jumper loop and bolted to the strain clamp body. In order to stabilize the jumper loop, the center of the loop is supported by a single suspension insulator string 3 through a saddle-type cable clamp 11 and an intermediate metal connector 12; see Figs. 2 and 3; which is pivotally attached to the cable clamp 11 by the pin 13 which allows freedom of movement of the clamp ll and cable. 9. This is only one illustrated application of the assembly shown in Figs. 2 and 3. It may also be used as a single suspension string at tangent points in the line, or at points where small angles in the line are involved.

v The line ends of the double-strain insulator strings} are shielded by grading shields 14 and the line end of the single suspension insulator string 3 is shielded by ,a combined grading and corona shield 15.

The details of the combined grading and corona shield 15 are shown most clearly in Figs. 2 and 3. The principal element of'the shield 15 is a unitary reentrantly curved tubular metallic member having two equal-length sub 'stantially straight transversely spaced and transversely aligned-parallel sides 16 connected by symmetrically -3 positioned normally downwardly and inwardly extending curved ends 17. A generally inverted U-shaped bracket 13 has the ends of its legs 1% attached correspondingly cit-center to the curved ends 1'7 in any suitable manner such as by welding and the yoke or center part 20 of the bracket 13 is generally parallel to the straight sides 16 of the shield and in the space between the straight sides 16 and the curved ends 17. The yoke or center part 20 of the bracket 18 is attached to the connector 12 in any suitable quick detachable manner such as by a bolt 21.

The shield 15 may be made of any suitable metal although aluminum alloy is preferred because of its lightness and resistance to corrosion and rusting.

As will be seen most clearly in Fig. 3, the shield 15 when viewed in a direction parallel to the cable conductor 9 and clamp 11 extends around said axis through an angle of between 270 and 360 with the connecting ends 17 below the cable conductor 9 and the spaced transversely aligned straight sides on opposite sides of and in substantially the plane of the lowermost insulator of the suspension insulator string 3. Thus, the straight sides 16 serve to grade the stress in the line end of'the insulator string 3 while the inwardly curved center parts of the ends 17 shield the ends and edges of the cable clamp 11 so as to remove the high stress concentrations from those ends and edges and thus inhibit corona discharge therefrom.

It will be seen from Fig. 3 particularly that by merely unscrewing bolt 21 and rotating the entire shield structure slightly counterclockwise about the axis of the cable conductor d that the shield can be dropped away from the line hardware at the lower end of the suspension insulator string 3 without in any way disturbing the supporting connections between the insulator string 3 and the cable conductor 9. Through the use of suitable hot line tools, this operation can, if necessary, be performed both as to the removal of a shield or the installation of a shield while the line is energized.

It will also be noted that the fastening bolt 21 goes through the connector 12 so that the weight of the shield 15 is born entirely by the connector 12 and its mass is not added to the mass of the cable clamp 11.

The construction of the shield 14 is shown most clearly in Figs. 4 and as consisting of a smoothly contoured curved length of metal tubing 22 which is normally maintained in the plane of the line end of the strain insulator strings 2 and which when viewed in a direction parallel to the axis of the cable conductor 5 and the strain insulator string 2 extends around the line end insulator or insulators 2 through an angle of between 270 and 360. The ends of the length of metal tube 22 are closed by metal plugs 23 having a hemispherically shaped outer surface for minimizing corona discharge therefrom. The shield 14 is attached to the yoke set 7 by a bracket of generally U-shaped construction having a yoke or center plate 2 to which are attached a pair of curved supporting bracket arms or legs 25 whose other ends are attached directly to the length of tube 22. Bolts 26 serve to attach the bracket plate 24 to the yoke set 7.

Due to the open construction of the shield 14, it may be attached to the yoke set 7 or removed therefrom Without breaking the mechanicahsupporting or strain-carrying connections between the strain insulator string 2 and the dead end clamps 6 by merely fitting the open-sided generally rectangular-shaped curved length of tubular material 22 over the connecting parts between the strain insulator string 2 and the dead end clamp 6.

While there has been shown and described a particular embodiment of the invention, it will be obvious to those skilled in the art that changes and modifications can be made without departing from the spirit and scope of the invention and it is therefore aimed in the appended claims to cover all such changes and modifications as fall Within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In an electric power transmission line for operation at alternating potentials between 300 and 500 kilovolts and at commercial frequencies, in combination, a cross arm of a metal supporting tower, a string of cap and pintype insulators attached at one end to said arm, a cable conductor, a metal clamp attached to said cable conductor, a metal connector between said clamp and the other end of said string of insulators, a pin-type pivotal connection between said connector and said cable clamp, and a shield for controlling the potential gradient in said string of insulators comprising a unitary circular cross section smoothly contoured curved metallic member, a metal bracket having two arms attached at spaced points to said unitary tubular metallic member, means for removably attaching said bracket to but one side of said connector, said tubular metallic member extending around the axis of said cable conductor through an angle of between 270 and 360, said tubular metallic member having a substantial portion of its extent in the plane of the insulator at the line end of said string of insulators.

2. In an electric power transmission line for operation at alternating potentials between 300 and 500 kilovolts and at commercial frequencies, in combination, a cross arm of a metal supporting tower, a string of cap and pintype strain insulatorsattached at one end to said arm, a cable conductor, a compression-type dead end metal clamp attached to said cable, a metal connector between said clamp and the other end of said strain insulators, a pin-type pivotal connection between said connector and cable clamp, said string of insulators and said cable conductor being substantially coaxial, and a shield for controlling the potential gradient through said string of insulators comprising a unitary circular cross section smoothly contoured curved tubular metallic member, a metal bracket having two arms attached at spaced points to said unitary tubular metallic member, means for removably attaching said bracket to one side of said connector, said tubular metallic member extending around the insulator at the line end of said string of insulators 'in a plane perpendicular to the axis thereof through an angle between 270 and 360.

3. In an electric power transmission line for operation at alternating potentials between 300 and 500 kilovolts and at commercial frequencies, in combination, a cross arm of a metal supporting tower, a double string of cap and pin-type strain insulators attached at one end to said arm, a cable conductor, a compression-type dead end metal clamp attached to said cable, a yoke set for interconnecting the other end of said double-strain insulator string and said cable clamp, the connection between said connector and said cable clamp being a pintype pivotal connection, said cable and double string of strain insulators being substantially coaxially aligned, and a shield for controlling the potential gradient in said double string of strain insulators comprising a unitary circular cross section smoothly contoured curved tubular metallic member in the form of an open-sided rec tangle, a metal bracket having two arms attached at spaced points to said unitary tubular metallic member, means for removably attaching said bracket to one side of said yoke set, said tubular metallic member extending around the axis of said double string of strain insulators through an angle between 270 and 360, said tubular metallic member having a substantial portion of its extent in the plane of the insulators at the line end of said double string of said strain insulators.

4. In an electric power transmission line for operation at alternating potentials between 300v and 500 kilovolts and at commercial. frequencies, in combination, a cross arm of a metal supporting tower, a vertical string of cap and pin-type suspension insulators attached at one end to said arm, a cable conductor, a metal clamp attached to said cable conductor, a metal connector between said clamp and the other end of said string of insulators, a pin-type pivotal connection between said connector and said cable clamp, the axis of said string of suspension insulators and said cable conductor being substantially mutually perpendicular, and a combined grading and corona shield comprising a reentrantly curved metal tube having two equal-length substantially straight transversely spaced and transversely aligned parallel sides connected by symmetrically positioned normally downwardly and inwardly extending curved ends, and a generally inverted- U-shaped bracket the ends of whose legs are respectively attached correspondingly off-center 'to said curved ends, the yoke part of said inverted-U-shaped bracket being in the space between said straight sides and curved ends, means for attaching the yoke part of said bracket to one side only of said connector, the line end of said insulator string being between said straight sides ofsaid shield and the downwardly and inwardly extending curved ends of said shield extending around and below the ends of said cable clamp.

5. In an electric power transmission line for operation at alternating potentials between 300 and 500 kilovolts and at commercial frequencies, in combination, a cross arm of a metal supporting tower, a string of cap and pintype strain insulators attached at one end to said arm, a cable conductor, a compression-type dead end metal clamp attached to said cable, a metal connector between said clamp and the other end of said string insulators, a pin-type pivotal connection between said connector and cable clamp, said string of insulators and said cable conductor being substantially horizontal, a shield for controlling the potential gradient through said string of strain insulators comprising a unitary circular cross section smoothly contoured curved tubular metallic member, a metal bracket having two arms attached at spaced points to said unitary tubular metallic member, means for removably attaching said bracket to one side of said connector, said tubular metallic member extending around the insulator at the line end of said string of insulators in a plane perpendicular to the axis thereof through an angle between 270 and 360", a string of cap and pin-type suspension insulators attached at one end to said arm and hanging vertically therefrom, a second cable conductor connected to said first cable conductor, a metal saddle clamp attached to said second cable conductor, a metal connector link between said saddle clamp and the other end of said vertical string of suspension insulators, a pin-type pivotal connection between said connector link and said saddle clamp, the axis of said vertical string of suspension insulators and said second cable conductor being substantially mutually perpendicular, and a combined grading and corona shield compris ing a reentrantly curved metal tube having two equal length substantially straight transversely spaced and transversely aligned parallel sides connected by symmetrically positioned normally downwardly and inwardly extending curved ends, and a generally inverted-U-shaped bracket the ends of whose legs are respectively attached correspondingly ofi-center to said curved ends, the yoke part of said inverted-U-shaped bracket being in the space between said straight sides and curved ends, means for attaching the yoke part of said bracket to one side only of said connector link, the line end of said vertical insulator string being between said straight sides of said combined shield and the downwardly and inwardly extending curved ends of said combined shield extending around and below the ends of said saddle clamp.

6. In an electric power transmission line for operation at alternating potentials between 300 and 500 kilovolts and at commercial frequencies, in combination, a cross arm of a metal supporting tower, a double string of cap and pin-type strain insulators attached at one end to said arm, a cable conductor, a compression-type dead end metal clamp attached to said cable conductor, a yoke set for interconnecting the other end of said doublestrain insulator string and said'cable clamp, the con'nelc titon between said yoke set and'said cable clamp being a pin-type pivotal connection, said cable and double string of strain insulators being substantially horizontally coaxially aligned, a shieldfor controlling the potential gradient'in said double'string of strain insulators comprising an open-sided generally rectangular-shaped length of-metal tube having four smoothly contoured curved corners, the ends of said tube which form the open side of the rectanglebeing closed by hemispherically shaped metal plugs, a bracket for attaching said curved tube to said yoke set comprising a metal plate connected to said curved tube by twin curved tubular members, said open sided rectangular-shaped length of tube surrounding the line end of said double string of strain insulators and be ing positioned by said bracket in the plane of the line end insulators, a vertical string-of cap and-pin-type'sus= pension insulators attached at one end to said arm, a second cable conductor connected to the first cable conductor, a metal saddle clamp attached to said second cable conductor, a metal connector between said saddle clamp and the other end of said vertical string of insulators, a pin-type pivotal connection between said connector and said saddle cable clamp, the axis of said vertical string of suspension insulators and said second cable conductor being substantially mutually perpendicular, a combined grading and corona shield comprising a reentrantly curved metal tube having two equal-length substantially straight transversely spaced and transversely aligned parallel sides connected by symmetrically positioned normally downwardly and inwardly extending curved ends, and a generally inverted-U-sl1aped bracket the ends of whose legs are respectively attached correspondingly off-center to said curved ends, the yoke part of said inverted-U-shaped bracket being in the space between said straight sides and curved ends, means for attaching the yoke part of said bracket to one side only of said conductor, the line end of said vertical insulator string being between said straight sides of said combined shield and the downwardly and inwardly extending curved ends of said shield extending around and below the ends of said saddle cable clamp.

7. A grading shield in combination with the line end of a high voltage double-strain insulator string provided with an interconnecting metallic yoke set comprising, an open-sided generally rectangular-shaped length of metal tube having four smoothly contoured curved corners, the ends of said tube which form the open side of the rectangle being closed by hemispherically shaped metal plugs, and a bracket for removably attaching said curved tube to said yoke set comprising a metal plate connected to said curved tube by twin curved tubular members, said curved tube positioned within a plane disposed normal to the axis of said insulator string and embracing said axis. I

8. A hot line mountable grading shield for the line end of a high voltage double-strain insulator string provided with an interconnecting metallic yoke set compris ing, an open-sided generally rectangular-shaped length of circular cross section aluminum alloy tube having four smoothly contoured corners, the ends of said tube which form the open side of the rectangle being closed by hemispherically shaped metal plugs, means for detachably connecting said tube with respect to said insulator string and yoke set whereby said tube is positioned in a plane normal to the axis of said insulator spring while embracing said axis comprising twin curved tubular supports attached to the tube opposite said open side and extending generally toward the center of said rectangle, a connecting plate attached to the ends of said twin tubular supports, and means for removably attaching said plate to said yoke set.

9. A combined grading and corona shield for hardware at the line end of a high voltage vertical suspension insulator string comprising, a closed reentrantly curved metal tube having two horizontally disposed equal-length substantially straight transversely spaced and transversely aligned parallel sides connected by symmetrically posiv ti'oned normally downwardly and inwardly extending rev entrantly curved ends, and a generally inverted-U-shaped bracket the ends of whose legs are respectively attached correspondingly off-center to said curved ends, the yoke part of said inverted-U-shaped bracket being in the space between said straight sides and curved ends and adapted for detachable connection to said hardware whereby the lowermost portions of: said reentrantly curved ends will be positioned below said hardware.

- 10. A hotline mountable combined grading and corona shield for hardware at the line end of a high voltage vertical suspension insulator string comprising, a closed reentrantly curved circular cross section aluminum alloy tube having two horizontally disposed equal-length substantially straight transversely spaced and transversely aligned parallel sides connected by symmetrically positioned normally downwardly and inwardly extending re,- entrantly curved ends, and a generally inverted-U-shaped bracket the ,ends of whose legsare respectively attached correspondingly off-center to said curved ends, the yoke part of said inverted-U-shaped bracket being in the space between said straight sides and curved ends and adapted for detachable connection to said hardware whereby the lowermost portions of said reentrantly curved ends will be positioned below said hardware.

References Cited in the file of this patent UNITED STATES PATENTS McCoy May 20, 1952 

